Concept: A method reference allows you to reference a method without executing it.
Relationship with lambda expressions: Both require a target type context composed of a compatible functional interface.
Functional instance: A method reference creates an instance of a functional interface when evaluated.
References to Static Methods
The class name and method are separated by a double colon (::), inserted in JDK 8.
Can be used wherever compatible with the target type.
Example of Reference to Static Methods
Functional interface:
Name: IntPredicate
Method: test(int value)
Parameters: an integer.
Return: a boolean (tests conditions on integer values).Auxiliary class:
Name: MyIntPredicates
Implemented static methods:
isPrime(): checks if the number is prime.
isEven(): checks if the number is even.
isPositive(): checks if the number is positive.Main method:
Name: numTest()
Parameters:
A reference to the IntPredicate.
interface An integer being tested.Execution:
In the main() method, three tests are performed with numTest().
For each test, a specific method reference (isPrime, isEven, isPositive) is passed as an argument for execution.
// Demonstrates a reference to a static method.
// Functional interface for numeric predicates that operates with
// integer values.
interface IntPredicate {
boolean test(int n);
}
// This class defines three static methods that check an integer
// in relation to some condition.
class MyIntPredicates {
// A static method that returns true when a number is prime.
static boolean isPrime(int n) {
if(n
for(int i=2; i
if((n % i) == 0)
return false;
}
return true;
}
// A static method that returns true when a number is even.
static boolean isEven(int n) {
return (n % 2) == 0;
}
// A static method that returns true when a number is positive.
static boolean isPositive(int n) {
return n > 0;
}
}
class MethodRefDemo {
// This method has a functional interface as its type
// first parameter. Therefore, you may receive a reference to
// any instance of this interface, including a created one
// by a method reference.
static boolean numTest(IntPredicate p, int v) {
return p.test(v);
}
public static void main(String args[])
{
boolean result;
// Here, a reference to the isPrime method is passed to numTest().
result = numTest(MyIntPredicates::isPrime, 17);
if(result) System.out.println("17 is prime.");
// Then a reference to the isEven method is used.
result = numTest(MyIntPredicates::isEven, 12);
if(result) System.out.println("12 is even.");
// Now a reference to the isPositive method is passed.
result = numTest(MyIntPredicates::isPositive, 11);
if(result) System.out.println("11 is positive.");
}
}
The reference to the static method isPrime() is passed as an argument to the numTest() method.
The isPrime() method is compatible with the IntPredicate functional interface.
The expression MyIntPredicates::isPrime creates a reference where isPrime() implements the test() method of IntPredicate.
The other calls to numTest() follow the same compatibility and operating principle.
Any method that has the boolean signature test(int n) can be used as a reference for the IntPredicate interface.
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